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`U8005216704A
`
`[11] Patent Number:
`
`5,216,704
`
`[45] Date of Patent:
`
`Jun. 1, 1993
`
`Attorney, Agent, or Firm—Eckert Seamans Cherin &
`Mellott
`
`[57]
`
`ABSTRACT
`
`First and second ISDN modems are disposed at a sub-
`scriber location and telephone system central office and
`are arranged to operate in a normal mode, and in a
`bypass mode upon power failure at the subscriber loca-
`tion. The modems are coupled through the telephone
`lines via interface transformers having centertaps. The
`centertap at the subscriber location is split and coupled
`to a sealing current source and that at the central office
`is coupled to a sealing current detector and ground.
`Sealing current flows between the respective transform-
`ers during normal operation. A drop in sealing current
`due to subscriber power failure is sensed at the central
`office, causing local logic to bypass the central office
`interface transformer and to couple the central office
`voice switch directly to the telephone lines. Loop cur-
`rent detectors at the subscriber location and at the cen-
`tral office determine whether the subscriber location
`
`voice telephone device is on-hook. If power is restored
`when on-hook,
`the local
`logic recouples the output
`interface transformer to the telephone lines and the
`voice telephone device is recoupled to the modem. At
`the same time, the central office detects whether sealing
`current has been restored and whether the subscriber
`voice device is on-hook, and recouples its output inter-
`face transformer to the telephone lines and the voice
`switch to the input
`interface of the central office
`modem, resulting in restoration of normal operation.
`
`10 Claims, 5 Drawing Sheets
`
`United States Patent
`Williams et a1.
`
`[19]
`
`[54] METHOD FOR REMOTE POWER FAIL
`DETECTION AND MAINTAINING
`CONTINUOUS OPERATION FOR DATA
`AND VOICE DEVICES OPERATING OVER
`LOCAL LOOPS
`
`[75]
`
`Inventors: Arthur B. Williams, Smithtown;
`David T. Lundquist, Stony Brook,
`both of N.Y.
`
`[73] Assignee: Coherent Communications Systems
`‘
`Corp., Hauppauge, NY.
`
`[21] App]. No.: 713,824
`
`[22] Filed:
`
`Jun. 12, 1991
`
`Int. Cl.5 ............................................ H04M 11/00
`[51]
`[52] US. Cl. ........................................ 379/93; 379/94;
`379/387; 379/413; 370/110.1
`[58] Field of Search ....................... 379/94, 2, 93, 387,
`379/413; 370/110.1
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`.
`
`2,572,027 10/1951 Horwitz .
`3.715.503
`2/1973 Jungbluth et a1.
`3,983,340
`9/1975 Lima et al.
`.
`4,076.961
`2/1978 Holsinger et a1.
`4,575.584
`3/1986 Smith et a1.
`,
`4,685,129
`8/1987 Gavrilovich ........................ 379/413
`4.797.874
`1/1989 Chahabadi et a1.
`....... 379/2
`
`...... 379/2
`..
`4,853.949
`8/1989 .Schorr et a1.
`
` 1/1991 Martinez ...........
`4.984.267
`379/413
`...................... 379/94
`5.034,948
`7/1991 Mizutani et al.
`
`,
`
`Primary Examiner—James L. Dwyer
`Assistant Examiner—Wing F. Chan
`
`24
`
`26
`
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`AVAYA INC. AV-1012
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`AVAYA INC. AV-1012
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`

`

`US. Patent
`
`June 1, 1993
`
`Sheet 1 of 5
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`US. Patent
`
`June 1, 1993
`
`Sheet 2 of 5
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`5,216,704
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`US. Patent
`
`June 1, 1993
`
`Sheet 4 of 5
`
`5,216,704
`
`124
`
`w4
`
`Sealing Current Detector
`
`Sealing Current Source
`
`Figure 3
`
`
`

`

`US. Patent
`
`June 1, 1993
`
`Sheet 5 of 5
`
`5,216,704
`
`Subscriber
`Location
`Logic #1
`
`Initialization
`
`
`
`£29
`
`Energize
`Relay 142
`
`Power
`Failure
`?
`
`
`
`
`
`
`
`
`
`
`De-energlze
`Relay 142
`
`
`
`.
`
`
`Power
`Restored
`
`
`408
`
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`
`
`On hack
`a
`
`
`
`Figure 4A
`
`406
`
`/
`
`,’
`
`,’
`
`
`
`.
`De-energize
`Relay 164
`
`Telephone
`System Central
`Office Location
`Logic #2
`
`Initialization
`
` Energize
`
`Relay 164
`
`
`
`
`
`Sealing
`current
`= 0
`9
`
`456
`
`Telephone
`On hook
`7
`
`458
`
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`
`
`
`
`
` Sealing
`current
`
`dele9cted
`
`
`Periodically
`Energize
`Relay 1 64
`
`Figure 4B
`
`

`

`1
`
`5,216,704
`
`METHOD FOR REMOTE POWER FAIL
`DETECTION AND MAINTAINING CONTINUOUS
`OPERATION FOR DATA AND VOICE DEVICES
`OPERATING OVER LOCAL LOOPS
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`This invention relates to the field of data and voice
`communications and more particularly to a method and
`system for maintaining voice communication in an
`ISDN type modem system during power fail condi-
`tions.
`2. Prior Art
`ISDN modems are a known means of combining
`voice and data communication over telephone lines.
`ISDN (for “Integrated Services Digital Network")
`modems are primarily digital devices which convert
`analog voice signals to digital data packets which are
`combined with other digital information data packets
`for communication over conventional telephone lines.
`Since ISDN modems transmit and receive voice signals
`via digital means, in many systems,.voice communica—
`tion becomes impossible when power is lost at a sub-
`scriber location.
`Prior systems have been developed to provide fail-
`safe voice communication when power is lost at a sub-
`scriber location. For example, in a digital communica-
`tion system described in US. Pat. No. 4,853,949—5-
`chorr et al., telephone system central office circuitry
`monitors loop current, loop DC continuity, incoming
`ringing from the telephone system central office switch
`and synchronization between the various digital devices
`in the system. Under normal conditions, the respective
`digital devices are in an idle state. In this state, although
`handshaking signals are exchanged, no voice or data
`link is established. When the subscriber location tele-
`
`the respective digital
`phone device(s) go off-hook,
`transceivers in the system become active and a full
`transmission path is established. Alternatively,
`if an
`incoming ringing signal
`is detected at the telephone
`system central office from the central office switch, the
`respective digital
`transceivers also become active.
`Under normal conditions, no DC path between the
`respective digital transceivers is established.
`When a power failure occurs at the subscriber loca-
`tion during a telephone call, a relay at the subscriber
`location bypasses the digital transceiver and the tele-
`phone device is directly coupled to the telephone line.
`As a result the telephone system central office digital
`transceiver loses synchronization. A DC continuity
`path is detected, and a relay connects the incoming loop
`directly to the telephone system central office switch.
`If a power failureoccurs during an idle state of the
`telephone device, DC continuity is not detected. Thus,
`the telephone system central office does not go into a
`bypass mode. If the telephone device subsequently goes
`off-hook during a power failure, the telephone system
`central office equipment senses the DC continuity and
`bypasses the telephone system central office digital
`transceiver, therefore connecting the subscriber tele~
`phone device to the central office switch.
`During a power failure, and with the telephone de-
`vice in an idle state, incoming ringing at the central
`office activates a ringing detector which will then in-
`struct the telephone system central office digital trans-
`ceiver to send a "wake~up" signal
`to the subscriber
`location transceiver. In this state, the subscriber loca-
`
`5
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`15
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`20
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`25
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`2
`tion digital transceiver cannot respond, eventually caus-
`ing the telephone system central office digital trans-
`ceiver to “time-out”, thus bypassing the telephone sys-
`tem central office digital transceiver and passing the
`ringing signal directly to the subscriber location. The
`subscriber location telephone then rings. If the sub-
`scriber location telephone device goes on-hook during a
`power fail mode, DC continuity will cease so the tele—
`phone system central office transceiver will no longer
`be bypassed.
`,
`During the idle state of the telephone, restoration of
`power at the subscriber location immediately activates a
`local relay, thus eliminating the bypass of the subscriber
`location transceiver. The telephone system central of-
`ficedigital transceiver remains connected in the idle
`state.
`
`If power is restored during a voice call, the bypass
`relay at the subscriber location is immediately activated,
`thus connecting the local transceiver which interrupts
`the loop current which is then sensed at the telephone
`system central office by the loop current detector. A
`bypass relay at the telephone system central office then
`changes state so that the telephone system central office
`digital transceiver is no longer bypassed. The telephone
`system central office digital transceiver then sends a
`“wake-up” signal to the digital transceiver at the sub-
`scriber location in an attempt to re-synchronizethe link.
`While achieving its stated purpose,
`the above de-
`scribed system has several problems. A standard re~
`quirement for local loop systems is to provide a DC
`sealing current to the loop. In the absence of sealing
`current, a poorly-conducting oxide film will develop
`over a period of time at the various non-soldered wire
`splices used in the system. A sealing current in the range
`of 10 mA is used to prevent oxidation and maintain low
`resistance across the non-soldered connections. The
`fact that the system of Schorr et a1. relies upon a lack of
`DC continuity during normal operation, prevents the
`introduction of beneficial sealing current. This violates
`ISDN standards as well as normal practice.
`_
`Further, in the system of Schorr et al., if power is
`restored during a voice call,
`the subscriber location
`bypass relay will immediately be activated and cause a
`current interruption by breaking the direct path over
`the local
`loop. Any momentary open-loop condition
`will be very disruptive and most likely results in a lost
`call.
`system, described in US. Pat. No.
`In another
`4,575,584—Smith et al., a combined digital and analog
`telephone is normally connected to a digital telephone
`exchange (PBX) and operates in a digital mode. In the
`event of a power failure or a telephone exchange mal.
`function, the digital telephone is converted to an analog
`mode,
`is disconnected from the digital
`telephone ex-
`change (PBX), and is connected to a local analog tele-
`phone line. When power is restored, the telephone re-
`verts to a digital mode and is recoupled to the digital
`telephone exchange. While this system also provides a
`degree of fail safe operation, the system is complicated,
`requiring both digital and analog telephone connections
`for operation.
`In view of the foregoing, no system is known for
`detecting power failure at a remote subscriber location
`having a digital multiplexer/modem and an associated
`analog telephone device wherein upon detection of a
`remote power failure, the respective digital circuitry of
`the system is bypassed and the remote analog telephone
`
`

`

`3
`is coupled directly to the telephone system central of-
`fice over an analog local loop. No prior system moni-
`tors power status of a remote subscriber location at the
`telephone system central office for the purpose of re-
`storing normal operation when power has returned.
`SUMMARY AND OBJECTS OF THE
`INVENTION
`
`In summary, the present invention contemplates a
`method and system for maintaining voice communica-
`tion in an ISDN modem system during power failure
`conditions. The present
`invention includes first and
`second ISDN modems disposed at a subscriber location
`and telephone system central office,
`respectively,
`wherein the subscriber location ISDN modem includes
`
`input interfaces coupled to a voice telephone device and _
`a data terminal and the telephone system central office
`location ISDN modem includes input interfaces cou-
`pled to a voice switch and a data processing device. The
`respective ISDN modems are coupled to conventional
`telephone lines with interface transformers having cen-
`tertap terminals. The split centertap terminals of the
`subscriber location interface transformer is coupled to a
`sealing current source and the centertap terminal of the
`telephone system central office location interface trans-
`former is coupled to ground through a sealing current
`detector such that a sealing current flows between the
`respective transformers during normal operation. If a
`power failure occurs at
`the subscriber location,
`the
`subscriber location voice telephone device is coupled
`directly to the telephone lines, and the current interface
`transformer is decoupled from the telephone lines by a
`local logic circuit, resulting in a drop in the flow of
`sealing current. The drop in sealing current is remotely
`sensed at the telephone system central office, causing
`local logic to bypass the telephone system central office
`interface transformer and to couple the voice switch
`directly to the telephone lines. Loop current detectors
`at
`the subscriber location and the telephone system
`central office monitor the system to determine when the
`subscriber location voice telephone device is on-hook.
`If power is restored at the subscriber location and if the
`voice telephone device is on-hook, the local logic cir-
`cuit recouples the output interface transformer to the
`telephone lines and the voice telephone device is recou-
`pled to the input interface of the subscriber location
`ISDN modem. At the same time, the telephone system
`central office detects whether sealing current has been
`restored and whether the subscriber location voice
`telephone device is on—hook. When this condition is
`detected, the output interface transformer of the tele-
`phone system central office is recoupled to the tele-
`phone lines and the, voice switch is recoupled to the
`input interface of the telephone system central office
`ISDN modem resulting in a restoration of normal sys-
`tem operation.
`Accordingly, it is an object of the present invention
`to provide a method and system for maintaining voice
`communication in a voice and data communications
`system during power fail conditions.
`It is another object of the present invention to pro-
`vide a fault tolerant voice and-data communications
`system which does not rely on power supplied from a
`telephone system central office.
`It is still another object of the present invention to
`provide the advantages of a remotely powered voice
`and data communications system without
`requiring
`
`IO
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`20
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`5,216,704
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`4
`hazardous voltages on the communication lines which
`couple the respective communication terminal sites.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The accomplishment of these and other objects may
`be fully understood through the following detailed de-
`scription of an exemplary embodiment and the accom-
`panying drawings, in which:
`FIG. 1A is a block diagram of one type of prior art
`ISDN modem system incorporating a local power feed
`at a subscriber site.
`FIG. 1B is a block diagram of another type of prior
`art ISDN modem system incorporating a phantom
`power feed from a telephone system central office.
`FIG. 2 is a block diagram of the preferred embodi-
`ment of the present invention.
`FIG. 3 is a schematic diagram of the sealing current
`source and sealing current detector described in con-
`junction with FIG. 2.
`FIGS. 4A and 4B are flow diagrams detailing the
`operation of the logic circuits which control the opera-
`tion of the subscriber location equipment and the tele-
`phone system central office equipment, respectively.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`ISDN modems are a known means for providing
`digital voice and information data communications over
`conventional
`telephone lines. Typically, ISDN type
`subscriber modems digitize voice signals originating
`from an analog voice telephone device and combine the
`digitized voice signals with information data for trans-
`mission to a telephone system central office. Con-
`versely, ISDN modems convert received data signals to
`analog voice signals and information data signals pro-
`cessed by local voice telephone devices and local data
`terminals. Since the voice transmission is digital, the
`conversion of an analog voice signal to a digital form,
`and vice versa, depends on the respective modems used
`at
`the subscriber location and the telephone system
`central office. Accordingly, voice telephone operation
`ceases when system power is lost. There are currently
`two basic methods for providing power for remote
`customer sites using ISDN type equipment. These
`methods are referred to as Local and Phantom feeds
`and are shown in block diagram form in FIGS. 1A and
`1B, respectively. Referring now to FIG. 1A, the local
`feed method is relatively simple and cost effective. One
`disadvantage in this type of system is that with loss of
`local power, telephone operation is completely inter-
`rupted, including voice operation. In the system 10, a
`telephone system central office 12 communicates with a
`subscriber location 14 through ISDN modems 16, 18
`which are coupled via conventional telephone lines 20,
`22. At the subscriber location 14, a conventional tele-
`phone 24 and a data terminal or processing device 26
`are interfaced to ISDN modem 18 wherein ISDN
`modem 18 converts the analog voice signal generated
`by telephone 24 to a digital signal which is combined
`with the information data signal used by data terminal
`or processing device 26. The central office ISDN
`modem 16 is coupled to conventional data processing
`means 28 and a conventional voice switch 30 wherein
`ISDN modem 16 converts the digital voice signal gen-
`erated by ISDN modem 18 to a analog voice signal
`processed by voice switch 30 and to the information
`data signal processed by processing device 28. In this
`system configuration, power for the ISDN modem 18 at
`
`

`

`5,216,704
`
`5
`the subscriber location 14 is provided by a local power
`supply 32 which is assisted by a conventional battery
`backup system 34. As noted above, one disadvantage
`for this type of system is the fact that under power fail
`conditions, communications for the entire system are 5
`interrupted. The battery backup system 34 provides
`some protection in this situation but storage batteries
`require periodic maintenance and have a finite life. In
`addition, storage batteries are expensive and the battery
`backup system 34 typically requires a DC to DC con- 10
`verter to generate the operating voltages required by
`the ISDN modem circuitry. An uninterruptable power
`supply may be substituted for the battery backup system
`but these types of power supplies are expensive and also
`require periodic maintenance. However,
`this system 15
`configuration is relatively simple to implement and does '
`not require potentially hazardous voltages to appear on
`telephone lines 22, 20.
`Another type of conventional ISDN modem system
`is the phantom power feed system 50 as shown in FIG. 20
`1B. For the sake of clarity, devices which perform
`identical functions as the devices shown in FIG. 1A
`
`bear identical designations. In the system 50, power for
`the ISDN modem 10’ is provided by the central office
`12’ via telephone lines 20‘ and 22'. Specifically, a power 25
`source 52, located in central office 12’ is coupled to
`power sink circuitry 54 located at the subscriber loca~
`tion 14’. This system configuration relies on the fact that
`even in the event of a power failure at the subscriber
`location 14', the telephone system central office 12' is 30
`always powered from either a local battery, the local
`.
`AC main power supply or an uninterruptable power
`supply (not shown). The telephone lines 20', 22' furnish
`power from the telephone system central office 12’ to
`the subscriber location 14'. In order to provide suffi— 35
`cient power to the subscriber location 14’, and to over-
`come the resistance of the local loop (coupled via tele-
`phone lines 20', 22), voltages as high as 140V DC must
`be generated by the telephone system central office 12’.
`In addition, a DC-DC converter (not shown) is required 40
`at the subscriber location 14' to generate the various
`voltages required to operate ISDN modem 18'. This
`system configuration is both costly and inefficient. Typ-
`ically, only 25% of the power provided at the telephone
`system central office 12’
`is actually delivered to the 45
`ISDN modem 18’ due to the inherent resistance of tele-
`phone lines 22', 20'. Furthermore, the 140 volt voltage
`present on telephone lines 22', 20’ may be considered
`hazardous in many situations. Accordingly, this type of
`system configuration is not preferred.
`Referring now to FIG. 2, the preferred embodiment
`of the present
`invention is shown in block diagram
`form. The present invention provides all the advantages
`of the system described in conjunction with FIG. 1A
`while also eliminating the problems which occur during 55
`power fail conditions. In the system 100, conventional
`ISDN modems 102, 104 are employed at the subscriber
`site 106 and the telephone system central site 108, recep-
`tively. As in the systems described above, the subscriber
`location ISDN modem 102 is interfaced to an analog 60
`voice telephone device 24 and some type of data termi-
`nal device 26. The telephone system central office
`ISDN modem 104 is interfaced to a data processing
`device 28 and a conventional voice switch 30. In the
`
`50
`
`ISDN 65
`invention,
`preferred practice of the present
`modems 102, 104 are interfaced to telephone lines 110,
`112 through transformers 114, 116, respectively. Each
`of respective transformers 114, 116 are provided with
`
`6
`centertap type terminals 118, 120, wherein split center-
`tap terminal 118 is coupled to a sealing current source
`124 and centertap terminal 120 is coupled to sealing .
`current detector 122. Sealing current detector 122 is
`connected to ground 170.
`The subscriber location 106 equipment is coupled to
`telephone lines 110, 12 through a plurality of switches
`126—132 which are configured as a conventional multi~
`pole, single-throw relay contacts which are controlled
`by relay coil 142. Relay coil 142 is selectively energized
`by subscriber location logic circuitry 144. The detailed
`operation of subscriber location logic circuitry 144 is
`discussed in detail below in conjunction with FIG. 4A.
`In the preferred practice of the present
`invention,
`switches 126, 132, 134, and 140 are normally open and
`switches 128, 130, 136 and 138 are normally closed. As
`shown in FIG. 2, telephone lines 110, 112 are coupled to
`transformer 114 through switches 128, 130,
`respecv
`tively. Telephone 24 has one terminal coupled to ISDN
`modem 102 through switch 136. This terminal of tele-
`phone 24 may also be coupled directly to telephone line
`110 through switches 126, 134. The other terminal of
`telephone 24 may be coupled to ISDN modem 102
`through conventional
`loop current detector 146 and
`switch 138. The output of loop current detector 146
`may also be coupled directly to telephone line 112
`through switches 132, 140. Switches 134—140 are also
`configured as conventional multi-pole, single-throw
`relay contacts which are controlled by relay coil 142.
`Loop current detector 146 also has a logic output cou-
`pled to subscriber location control logic 144 wherein
`the logic output of loop current detector indicates
`whether telephone device 24 is off-hook.
`The telephone system central office location 108
`equipment
`is coupled to telephone lines 110, 112
`through a plurality of switches 148—162 which are con-
`figured as a conventional multi-pole, single-throw relay
`contacts which are controlled by relay coil 164. Relay
`coil 164 is selectively energized by telephone system
`central office location logic circuitry 166. The detailed
`operation of telephone central office location logic cir-
`cuitry 166 is discussed in detail below in conjunction
`with FIG. 4B. In the preferred practice of the present
`location, switches 148, 154, 156 and 162 are normally
`open and switches 150, 152, 158 and 160 are normally
`closed. As shown in FIG. 2, telephone lines 110, 112 are
`coupled to transformer 116 through switches 150, 152,
`respectively. Voice switch 30 has one terminal coupled
`to ISDN modem 104 through switch 158. This terminal
`of voice switch 30 may also be coupled directly to tele-
`phone line 110 through switch 156, loop current detec-
`tor 168 and switch 148. The other terminal of voice
`
`switch 30 may be coupled to I SDN modem 104 through
`switch 160, or may be coupled directly to telephone line
`112 through switches 154, 162. Loop current detector
`168 also has a logic output coupled to telephone system
`central office logic 166 wherein the logic output of loop
`current detector 168 indicates whether the telephone
`device 24 is off-hook.
`Each of the respective switches 126—140 and 148—162
`are shown in their normal state wherein relay coils 142,
`164 are energized. In this state, the ISDN modems are
`coupled via telephone lines 110, 112 through transform-
`ers 114, 116. Sealing current flows from sealing current
`source 124, and transformer 114 to the centertap termi-
`nal 120 of transformer 116 to sealing current detector
`122 and ground 170. The respective ISDN modems 102,
`104 function normally and telephone device 24 and
`
`

`

`7
`terminal device 26 communicate with the telephone
`system central office 108 via the conventional ISDN
`protocol, as controlled by modems 102, 104.
`The system 100 of the present
`invention provides
`continuous voice telephone operation in situations
`wherein power is lost as the subscriber location 106,
`although the system 100 could also be arranged to ac-
`commodate situations wherein power is lost at either
`the subscriber location 106, or the telephone system
`central office 108. The detailed operation of the system
`100 may be fully understood by referring to FIGS. 2,
`4A and 4B. The logic circuits 144, 166 may be of a
`variety of conventional
`types such as combinational
`logic circuits or microprocessors operating in accor-
`dance with the routines shown in FIGS. 4A and 4B.
`
`When the logic circuits 144 and 166 are initialized, the
`respective relays 142, 164 are energized, as represented
`by steps 402 and 452,
`thus placing the respective
`switches 126—140 and 148—162 in the position shown in
`FIG. 2. A power fail condition at the subscriber loca-
`tion 106 is detected at decision 404 by subscriber loca-
`tion logic 144. As soon as a power fail condition occurs,
`relay 142 is de-energized at step 406. This causes tele-
`phone device 24 to be coupled directly to telephone
`lines 110, 112 through switches 126, 134 and through
`loop current detector 146 and switches 132, 140, respec-
`tively. In this state, ISDN modem 102 and terminal
`device 26 ar switched out of the system. However, since
`power is temporarily not available at
`the subscriber
`location 106, this change is inconsequential.
`When power is lost at the subscriber location 106, and
`transformer 114 is switched out of the telephone loop,
`the power fail condition is remotely detected at
`the
`telephone system central office 108 by sealing current
`detector 122. Since transformer 114 has been switched
`
`out of the loop, the sealing current source 124 is lost and
`the sealing current drops to zero. This change is de-
`tected at decision 454 by sealing current detector 122.
`When the sealing current detector 122 detects the drop
`in sealing current, the logic output of sealing current
`detector 122 changes state. When the logic output of
`sealing current detector 122 changes state, logic circuit
`166 de-energizes relay coil 164 at step 456, thus cou-
`pling voice switch 30 directly to telephone lien 110
`through switch 156,
`loop current detector 168 and
`switch 148, and to telephone line 112 through switches
`154, 162.
`If power has been restored as determined at decision
`408 and the telephone device is on-hook at decision 410
`as indicated by loop current detector 146, logic circuit
`144 re-energizes relay 142, as indicated in step 402. At
`the same time, logic circuit 166 determines whether the
`telephone system 24 is on-hook, as determined at deci-
`sion 458 by checking the output of loop current detec-
`tor 168. If telephone device 24 is on-hook, logic circuit
`166 periodically energized relay 164 at step 460 to deter-
`mine whether sealing current is flowing at decision 462,
`in particular by sensing the output of sealing current
`detector 122. If sealing current is detected, logic circuit
`166 energizes relay 164 to return the system 100 to its
`normal state at step 452.
`Referring now to FIG. 3, the sealing current source
`124 and the sealing current detector 122 are shown in
`schematic form. The sealing current source 124 as
`shown is based on an NPN transistor 302 having an
`emitter coupled to a negative voltage such as a 48 volt
`power source thro'ugh resistor 304. The negative 48 volt
`power source may be of the type available in most con-
`
`10
`
`15
`
`20
`
`25
`
`3O
`
`35
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`40
`
`45
`
`50
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`55
`
`60
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`65
`
`5,216,704
`
`8
`ventional ISDN modern systems. The base of transistor
`302 is coupled to ground through resistor 308. Voltage
`limiting (Zener) diode 306 is coupled across the base-
`emitter junction of transistor 302. Resistor 304 and
`diode 306 are selected to provide a current capacity of
`approximately 10 mA through sealing current source
`124.
`Resistors’ 320 and 322 in series with the sealing cur-
`rent source path assure that the sealing current divides
`evenly between the two windings of transformers 114
`to prevent saturation. In addition after a condition of
`power failure when power is restored at the subscriber
`end, these resistors limit the current that transformer
`114 would normally drain from the central office loop
`since the central end is still bypassed until ISDN opera-
`tion is established. This prevents false off-hook detec-
`tion by loop current detector 168.
`Capacitor 324 bypasses resistors’ 320, 322 for ISDN
`transmission signals so no AC losses are incurred. The
`sealing current detector 122 comprises a conventional
`optical isolator 310 having an output transistor 312 and
`an isolated input, formed by cross-coupled light emit-
`ting diodes 314, 316, coupled to the centertap terminal
`of transformer 116. The other terminal of cross-coupled
`light emitting diodes 314, 316 is coupled to the collector
`of current source transistor 302, wherein whenever
`current flows into the centertap terminal of transformer
`116,
`through cross-coupled light emitting diodes,
`to
`ground, the collector of transistor 312 is driven low,
`thus indicating the presence of sealing current. In the
`absence of sealing current, the collector of transistor
`312 is pulled high by resistor 318.
`In summary, an improved system for maintaining
`voice communication in an ISDN modem system has
`been described. Accordingly, other uses and modifica-
`tions of the present invention will be apparent to per-
`sons of ordinary skill and all of such uses and modifica-
`tions are intended to fall within the scope of the ap-
`pended claims.
`We claim:
`
`1. An improved system for maintaining voice com-
`munications in a voice and data communications sys-
`tem, said system comprising:
`first and second voice and data modem means dis-
`posed at a subscriber location and a central office
`location, respectively, wherein each of said respec-
`tive voice and data modem means include voice
`and data input means and output means;
`first and second transformer means for coupling the
`output means of said respective first and second
`modem means to telephone lines, said first and
`second transformer means each including a wind-
`ing having a centertap terminal, the centertap ter-
`minal of said first transformer means being split and
`coupled to a sealing current source and the center-
`tap terminal of said second transformer means
`being coupled to ground;
`means for detecting a power failure at said subscriber
`location; and,
`-
`means for bypassing said first and second voice and
`data modem means and said first and second trans-
`former means upon detecting power failure at said
`subscriber location.
`2. The system of claim 1, wherein a sealing current
`detector is coupled between ground and the centertap
`terminal of said second transformer means.
`3. The system of claim 2, wherein a power failure at
`said subscriber location is detected by a lack of sealing
`
`

`

`9
`current flowing between said first and second trans-
`former means as indicated by said sealing current detec-
`tori
`
`5,216,704
`
`10
`a first input interface coupled to a voice telephone
`device with first selective coupling means and a
`second input